In a dirt course of a horse racing track, a river sand layer is normally formed on a subgrade layer in consideration of drainage and shock absorption, and the river sand layer is used as a cushion layer of the ground surface. However, such a cushion layer has the following disadvantages. That is, when it is used for a long term, the river sand is pulverized by kicking impacts of horseshoes, and the pulverized river sand is sedimented at the bottom portion of the cushion layer to form a hard plate-shaped layer. Therefore, water permeability is lowered and a drainage trouble occurs. Further, the substantial thickness of the cushion layer is reduced, resulting in occurrence of such an accident that horse legs are damaged when the horse kicks the hard plate-shaped layer with the horseshoes.
Therefore, it has been proposed that chipped rubber (which is mainly composed of wasted tire rubber) or plastic which are adopted in general sport facilities such as tracks for athletic sports, etc. are mixed with river sand to form a cushion layer of the dirt course.
However, such chipped rubber or plastic has a specific gravity of about 1.1 to 1.5, and when it is blended with sand, the chips are floated to the surface of the cushion layer because of the difference of the specific gravity therebetween, so that uniformity of the cushion layer cannot be kept. Further, much consideration has not been paid to the function of preventing pulverization (silting) of sand for its use term.
The present invention has been implemented on the basis of the above situation, and has an object to promote uniformity and stability of a cushion layer in the structure of the ground surface portion and also suppress pulverization of the cushion layer.
Further, the present invention has another object to provide the structure of the ground surface portion which can promote athletes or other ground players to have excellent motions (that is, which can give excellent driving force for walking or running: so-called good "kickback").